Location: Wisconsin....about 100 miles south of the Frozen Tundra and 70 miles east of Camp Randall
No, it is not true. Each cruise line can decide what the drinking age is onboard the ship. In ports, it's another matter. Carnival is 21. Princess is 21. Royal Caribbean is 21, but 18 - 20 year olds can buy and drink beer and wine with the parents written consent. I'm not sure about the other cruise lines.
Carnival Breeze with Ray Band Aerogirl 5/4/14!
Celebrity has the same policy as Royal Caribbean since they are owned by RCI.
Explorer of the Seas October 2013
Caribbean Princess July 2006, May 2010 & November 2012
Monarch of the Seas November 2008
Crown Princess November 2007
Celebrity Zenith November 2005
Enchantment of the Seas August 2004